Conventionally, technologies for controlling audio signal beams (i.e., sound waves converted into beams having directivities) by use of array speakers, in which plural speaker units are regularly arranged, are known. For example, Japanese Unexamined Patent Application Publication No. H03-159500 and Japanese Unexamined Patent Application Publication No. S63-9300 disclose technologies regarding array speaker systems.
A control method for sound directivity in an array speaker will be described with reference to FIG. 7.
In FIG. 7, reference numerals sp-1 to sp-n designate speaker units that are linearly arranged with prescribed distances therebetween. In the case of generation of an audio signal beam emitted towards a focal point X, a circle Y whose radius matches a distance L from the focal point X is drawn, and delay times (=Li/speed of sound (340 m/s)) are calculated in response to distances Li between the speaker units sp-i (where i=1, . . . , n) and the intersection points, at which the circle Y intersects line segments interconnecting between the focal point X and the speaker units sp-1 to sp-n respectively, and wherein they are applied to input signals of the speaker units sp-i. Thus, it is possible to control the sound directivity of the array speaker in such a way that audio signal beams respectively emitted from the plural speaker units sp-1 to sp-n reach the focal point X at the same time.
FIG. 8 is an illustration showing an example of the relationship between the focal point and sound directivity, and it shows a contour distribution of sound pressure energy with respect to a single frequency signal when plural speaker units are arrayed in an X-axis direction about the zero-centimeter-position of the X-axis. As shown in FIG. 8, it is possible to produce an intense sound directivity in a direction towards a focal point designated by a symbol “x ”.
As an application of this technology, there is provided a technology in which different sound directivities are imparted to different content so as to realize hearing of different content in the left and right of a room respectively. This technology is disclosed in Japanese Unexamined Patent Application Publication No. H11-27604, for example.
In general, audio signals have a wide range of frequency components within audio frequencies ranging from 20 Hz to 20 kHz. Such a frequency range matches a range of wavelengths ranging from 17 m to 1.7 cm. In the practical form of an array speaker, the sound directivity control is performed in such a way that audio signal beams emitted from plural speaker units may reach a specific focal point with the same phase. This indicates that at the focal point, audio signal beams converge at the same phase irrespective of frequencies of audio signals; hence, audio signal beams may be emphasized. In contrast, audio signal beams may converge substantially at the same phase at different positions outside of the focal point because of different wavelengths, which differ in response to frequencies thereof. That is, there occurs a phenomenon in which sound directivity differs in response to frequency.
FIG. 9 shows a simulation result with regard to sound directivity for a single frequency signal of 1 kHz; and FIG. 10 shows a simulation result with regard to sound directivity for a single frequency signal of 2 kHz. The same focal point is set in FIGS. 9 and 10.
In comparison between FIG. 9 and FIG. 10, it is obvious that when similar sound directivity control is performed with respect to a prescribed focal point, the sound directivity becomes intense (so as to form a sharp contour distribution of sound pressure energy) as frequencies become higher.
The aforementioned differences of sound directivity indicate that at any position outside of the focal point, source audio signals become out of balance in frequencies. At a position distant from the focal point, it is possible to realize hearing of low-frequency sound to some extent; however, hearing of high-frequency sound may be rapidly damped. Essentially, sound directivity control increases sound pressure energy at the focal point but decreases sound pressure energy at the other positions. In the practical form of an application, it is necessary for sweet spots allowing audio signals to be appreciated with a certain level to have appropriate areas. For this reason, it is preferable that a similar sound directivity distribution be applied to both of the high-frequency sound and low-frequency sound to some extent.
This invention is made in consideration of the aforementioned circumstances; hence, it is an object of the invention to provide an array speaker system having good sound directivity.